Since mid-April 2009, CDC, state and local health authorities in the United States, the World Health Organization (WHO), and health ministries in several countries have been responding to an outbreak of influenza caused by a novel influenza A (H1N1) virus. In March and early April 2009, Mexico experienced outbreaks of respiratory illness subsequently confirmed by CDC and Canada to be caused by the novel virus. The influenza strain identified in U.S. patients was genetically similar to viruses isolated from patients in Mexico. Since recognition of the novel influenza A (H1N1) virus in Mexico and the United States, as of May 6, a total of 21 additional countries had reported cases, with a total of 1,882 confirmed cases worldwide. Several WHO member states are conducting ongoing investigations of this worldwide outbreak, and WHO is monitoring and compiling surveillance data and case reports. On April 29, WHO raised the level of pandemic alert from phase 4 to phase 5, indicating that human-to-human spread of the virus had occurred in at least two countries in one WHO region. This report provides an update of the initial investigations and spread of novel influenza A (H1N1) virus worldwide.